1. Field of the Invention
The present invention relates to the biological control of fungal potato diseases. More particularly, this invention relates to the selection and use of bacterial isolates which are effective antagonists against fungal species responsible for potato dry rot and other potato diseases which occur in the field or in postharvest storage.
2. Description of the Art
The potato is the most important dicotyledonous source of human food, ranking as the fifth major food crop of the world. Fusarium-induced potato dry rot is an economically important problem of potatoes both in the field and in storage. Several species of the Fusaria induce this disease, however, Gibberella pulicaris (Fries) Sacc. (anamorph: Fusarium sambucinum Fuckel) is a major cause worldwide, especially in North America. Fusarium spp. can survive for years in field soil, but the primary inoculum is generally borne on seed tuber surfaces. The dry rot fungi infect potatoes via wounds in the periderm inflicted during harvesting or subsequent handling. In stored potatoes, dry rot develops most rapidly in high relative humidity (.gtoreq.70%) and at 15.degree.-20.degree. C., but continues to advance at the coldest temperatures safe for potatoes. Although rots caused by Fusarium seldom reach epidemic proportions, the level of infected tubers in storage often reaches 60% or higher, with average losses estimated in the 10-20% range. In addition to destroying tissue, F. sambucinum can produce trichothecenes that have been implicated in mycotoxicoses of humans and animals.
The high value of the potato crop and the significant economic losses caused by potato dry rot have led to investigations of various methods to control the disease. Success has been attained by use of the fungicides, thiabendazole and 2-aminobutane, which are applied to tubers at harvest or at preplanting [S. F. Carnegie et al., Ann Appl. Biol. 116: 61-72 (1990); S. S. Leach, "Control of Postharvest Fusarium Tuber Dry Rot of White Potatoes," pages 1-7 In ARS-NE-55, U.S. Dep. Agric., Washington, D.C.]. However, strong concerns are being raised about the potential adverse impact of these chemicals on ground and surface water reservoirs and on the health of agricultural product workers and consumers. Also, thiabendazole-resistant strains of F. sambucinum have emerged in populations from severely dry-rotted tubers in North America and in Europe. Potato breeding programs have given increased attention to development of cultivars with resistance to Fusarium, but most of the reported cultivars produced by these programs are resistant to only one or two of several Fusarium strains [S. S. Leach et al., Phytopathology 71(6): 623-629 (1981)].
One alternative to chemical fungicides in controlling potato rot is the use of biological agents. Postharvest biological control systems of fruit have been actively investigated for the past decade. These include iturins as antifungal peptides in biological control of peach brown rot with Bacillus subtilis [C. G. Gueldner, et al., Journal of Agricultural and Food Chemistry 36:366-370 (1988)]; postharvest control of blue mold on apples using Pseudomonas spp. isolate L-22-64 or white yeast isolate F-43-31 [W. J. Janisiewicz, Phytopathology 77:481-485 (1987)]; control of gray mold of apple by Cryptococcus laurentii [R. G. Roberts, Phytopathology 80: 526-530 (1990)]; biocontrol of blue mold and gray mold on apples using an antagonistic mixture of Pseudomonas sp. and Acremonium breve [W. J. Janisiewicz, Phytopathology 78:194-198 (1988)]; control of gray mold and reduction in blue mold on apples and pears with an isolate of Pseudomonas capacia and pyrrolnitrin produced therefrom [W. J. Janisiewicz and J. Roitman, Phytopathology 78:1697-1700 (1988)]; postharvest control of brown rot in peaches and other stone fruit with the B-3 strain of Bacillus subtills [P. L. Pusey and C. L. Wilson, Plant Disease 68:753-756 (1984)]; P. L. Pusey et al., Plant Disease 72:622-626 (1988)] and U.S. Pat. No. 4,764,371 to Pusey et al.); antagonistic action of Trichoderma pseudokoningii against Botrytis cinerea Pers. which causes the dry eye rot disease of apple [A. Tronsmo and J. Raa, Phytopathol. Z. 89:216-220 (1977)] and postharvest control of brown rot and alternaria rot in cherries by isolates of Bacillus subtilis and Enterobacter aerogenes [R. S. Utkhede and P. L. Sholberg, Canadian Journal of Microbiology 963-967 (1986)]. A review of biological control of postharvest diseases of fruits and vegetables is given by Wisniewski et al. [HortScience, 27:94-98 (1992)].
Efforts have also been made to isolate bacterial agents for controlling diseases and weeds affecting yields of gramineous crops. In U.S. Pat. No. 4,456,684, Weller et al. disclose a method for screening Pseudomonas fluorescens strains which are effective in suppressing Gaeumannomyces graminis var. tritici (Ggt) in field-grown crops and turf grass. Elliott et al. have isolated non-fluorescent Pseudomonas bacteria useful in the selective control of downy brome in small grain crop fields (U.S. Pat. No. 5,030,562). Similarly, Kennedy et al. teach the screening for bacterial strains for controlling jointed goat grass, a weedy pest in small grain crops (U.S. Pat. No. 5,163,991).
Reports have been made in the literature regarding the application of Pseudomonas spp. as antagonists against organisms responsible for degradation of potatoes. Curiously enough, worldwide interest in these gram-negative bacteria was sparked by 1970s research in which strains of P. fluorescens and putida, applied to seed pieces, improved the growth of potatoes [T. J. Burr et al., Phytopathology 68: 1377-83 (1978)]. Later studies documented that fluorescent pseudomonads increased the yield of potato 5-33% [M. N. Schroth et al., Science 216: 1376-81 (1982)]. Colyer et al. report partial control of soft rot development in potato tubers by a Pseudomonas putida isolate antagonistic to soft-rotting Erwinia spp. [Plant Disease, 68:703-705 (1984)]. De La Cruz et al. disclose the isolation of three strains of fluorescent pseudomonads from potato underground stems antagonistic against 30 strains of the ring rot bacterium Clavibacter michiganensis subsp. sepedonicus [Applied and Environmental Micro., 58:1986-91 (1992)]. Presently, there are no biocontrol systems for potato diseases in widespread commercial use.